It is standard practice in the oil and gas industry to nondestructively inspect tubular members that are to be used in a wellbore. Failure of such tubular members may result in drilling delays, production losses, environmental damage, and destruction of drilling and production equipment.
There are two primary methods of nondestructively inspecting tubular members for use in oil and gas well operations. The most common method is the magnetic particle method, which is accomplished by magnetizing the wall of the tubular member, typically a pipe, or by providing it with a residual magnetic field and thereafter dusting fine particles of iron or iron oxide on a region of the wall. The pattern of the particles indicates whether or not there are any flaws in the wall. While this method is sometimes adequate for inspection of the exterior surface of the pipe or for the interior surface near the ends of the pipe, it is not adequate for inspecting the interior surface of the pipe over its entire length. This results from the fact that the magnetic particle inspection method requires immediate access to the area being inspected, both for dusting the iron particles on the pipe, and for visually inspecting the pipe for indication of flaws.
A second recent method for nondestructive inspection of pipes is the electromagnetic flux leakage technique. Most commonly, this technique is applied by using an apparatus in which the magnetizing means and magnetic sensing means are mounted external to the pipe. The apparatus is placed in proximity to the exterior surface of the pipe and then moved about the exterior surface for inspection. The signals from the magnetic sensing means are generally displayed in analog form, such as on a strip chart recorder. Such an apparatus is disclosed in U.S. Pat. No. 4,510,447 to M. C. Moyer, et al. The apparatus is less sensitive to flaws on the inner surface of the pipe than to flaws on the outer surface of the pipe. This is especially true for thick walled pipe.
Various other apparatus have been suggested for detecting flaws on the inner surface of a pipe. One such apparatus is disclosed in U.S. Pat. No. 2,308,159 to Drummond et al. Drummond discloses a tubing defect detector which includes a drive motor mounted for travel along rails. Attached to the motor is a long shaft at the end of which is attached a single sensor core and a single coil, which are used for detecting flaws. For inspection operations a current is applied to the pipe to produce a closed magnetic field. The drive motor is energized to rotate the attached shaft, and the core and coil attached thereto. The motor is then pulled forward on the track, thereby causing the magnetic detection apparatus to move into the pipe being inspected. If there is a flaw in the pipe, an electric current is generated in the single coil of the sensing apparatus, and then indicated on a device such as an oscilloscope.
The Drummond apparatus has certain disadvantages. The motor used to rotate the apparatus is often too large to be placed inside the pipe being inspected. Thus, the rotating drive shaft must frequently be as long as the pipe to be inspected, creating a need for a powerful motor that can be driven through the length of the pipe. The coil used for detection of magnetic flux generally indicates that a flaw is present, but no further information is collected. Thus no details as to the exact location, axial or circumferential, or severity of the flaw is available, making manual inspection and repair logistically difficult. Drummond discloses no means which allows the apparatus to be adapted for use in pipes of varied sizes.
Other apparatus for detecting flaws located on the inner surface of a pipe are disclosed in U.S. Pat. Nos. 4,217,548 issued Aug. 12, 1980 to Furukawa et al. and 4,258,318, also to Furukawa et al. Furukawa discloses an apparatus for magnetically detecting flaws on the inner surface of a pipe comprising a magnetizing assembly supported exterior to the pipe being inspected and a detecting assembly supported from a support rod which is inserted in the interior of the pipe. Because neither the magnetizing means nor the detection means is rotatable, Furukawa discloses apparatus for rotating the pipe during inspection.
Because the apparatus disclosed in both Furukawa patents magnetize pipes from the exterior of the pipes, adequate magnetization on the internal surface of the pipe cannot be ensured. This concern is especially acute regarding thick walled pipes. The pipe must be rotated, which requires significant power, and may cause severe vibration. Further, neither Furukawa patent discloses any means for detailing the axial or circumferential location, or the severity, of the flaws. Again, manual inspection and repair is logistically difficult.